The invention relates to exhaust gas sensors, and more particularly to stoichiometric exhaust gas sensors.
Exhaust gas sensors are well known in the automotive industry for sensing the oxygen, carbon monoxide, or hydrocarbon content of the exhaust stream generated by internal combustion engines. Stoichiometric or xe2x80x9cNernstxe2x80x9dxe2x80x94type oxygen sensors (a widely-used type of exhaust gas sensor) measure the difference between the partial pressure of oxygen found in the exhaust gas and oxygen found in the atmosphere (reference side). By determining the amount of oxygen in the exhaust gas, the oxygen sensor enables the engine control unit to adjust the air/fuel mixture and achieve optimal engine performance.
Prior-art stoichiometric exhaust gas sensors typically include a cup-shaped sensing element or an elongated, multi-layered, bar-shaped sensing element that is supported in a housing. The sensing element typically includes a ceramic substrate, such as zirconium dioxide, that supports electrodes, heating elements, and the associated electrical leads. When the assembled sensor is mounted in the exhaust line, the ceramic element protrudes into the exhaust stream so that the exhaust-side electrode is directly exposed to and oriented substantially perpendicularly to the flow of exhaust gases. The reference-side electrode is isolated from the exhaust gas in an air-tight manner.
Zirconium dioxide, stoichiometric exhaust gas sensors can be contrasted with other types of exhaust gas sensors that operate using different fundamental principles. For example, one other well-known type of exhaust gas sensor is an amperometric or limiting current exhaust gas sensor. A limiting current sensor includes a small ceramic cavity that is hermetically sealed onto a flat solid oxide electrolyte slab. The cavity has a small hole that allows molecular oxygen to diffuse into the cavity from the environment when a DC bias is applied across the electrolyte to remove oxygen from the cavity through the solid electrolyte. A limiting current scenario is eventually reached and is governed by the rate of viscous diffusion of molecular oxygen from the environment into the cavity through the small hole. The concentration of oxygen is related linearly to the limiting current since the diffusion rate through the hole is governed by the partial pressure of oxygen in the environment outside the cavity.
Another known type of exhaust gas sensor is a titanium dioxide exhaust gas sensor. Titanium dioxide is a transition metal oxide that undergoes a change in its electrical resistance depending on the content of oxygen in the exhaust gases. The titanium dioxide (titania) is used in the form of a microscopically porously fired layer so that the exhaust gas can freely permeate into and through the mass of titania. By measuring the change in resistance of the titania, the air/fuel mixture can be maintained for optimal engine performance.
The present invention provides an improved stoichiometric exhaust gas sensor having a sensing element that is smaller and less expensive to manufacture than the prior-art cup-shaped or elongated, bar-shaped sensing elements. The smaller sensing element enables the overall size of the sensor assembly to be reduced. The sensing element is in the form of a flat disk or a flat polygonal-shaped plate (hereinafter referred to only as a disk). The exhaust-side electrode is formed on one side of the disk and the remaining components are all formed on the opposite side of the disk. A hole in the disk provides for the air-tight electrical connection between the exhaust-side electrode and the reference-side of the ceramic element. All of the electrical contacts are therefore formed on the reference-side of the disk, and electrical contact is simplified using spring-biased pin connectors or other suitable connectors supported in the housing. The configuration of the disk and the design of the housing permits the disk to be oriented substantially parallel to the flow of exhaust gases, thereby reducing the exposure of the exhaust-side electrode to water and particles that would otherwise strike the exhaust-side electrode and potentially cause poisoning and/or thermal shock problems. Sealing and insulating the disk with respect to the housing is also greatly facilitated.
More specifically, the invention provides a sensor element for an exhaust gas sensor. The sensor element includes a support member having an exhaust side, a reference side, and an aperture extending through the support member between the exhaust side and the reference side. The sensor element also includes an exhaust-side electrode on the exhaust side of the support member. The exhaust-side electrode is electrically connected to a contact on the reference side of the support member via a lead extending through the aperture. The aperture is sealed around the lead such that gas cannot pass through the aperture from the exhaust side to the reference side of the support member.
The invention also provides an exhaust gas sensor for sensing a gas in a flow of exhaust gases. The sensor includes a housing and a sensor element supported by the housing. The sensor element includes a support member having an exhaust side, a reference side, and an aperture extending through the support member between the exhaust side and the reference side. The sensor element further includes an exhaust-side electrode on the exhaust side of the support member. The exhaust-side electrode is electrically connected to a contact on the reference side of the support member via a lead extending through the aperture. The aperture is sealed around the lead such that gas cannot pass through the aperture from the exhaust side to the reference side of the support member.
In one aspect of the invention, the support member is oriented such that a substantially planar surface defined by the exhaust side is substantially parallel to the flow of exhaust gases when the exhaust gas sensor is installed on a vehicle. In another aspect of the invention, the sensor further includes a contact pin in the housing and engaged with the contact. The contact pin is biased toward the contact to maintain electrical contact with the contact. In yet another aspect of the invention the support member has a perimeter, and the contact pin is biased toward the contact to bias the support member against a portion of the housing such that exhaust gases cannot flow around the perimeter of the support member to the reference side.
The invention also provides a method of manufacturing a sensor element for an exhaust gas sensor. The method includes providing a support member having first and second sides, forming an aperture that extends between the first and second sides in the support member, forming a conductive lead that extends through the aperture, and forming an electrode on the first side of the support member and in electrical contact with the lead.
In one aspect of the invention, forming the conductive lead further includes sealing the aperture around the lead such that gas cannot pass through the aperture. In another aspect of the invention, the method further includes forming an electrode on the second side of the support member so that the electrode on the second side is electrically isolated from the lead and the electrode on the first side of the support member. In yet another aspect of the invention, the method further includes forming a heating element on the second side of the support member so that the heating element is electrically isolated from the lead and the electrodes.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings.